This invention relates to vacuum pumps and in particular to oil free or dry mechanical vacuum pumps.
A typical oil free or dry pump, i.e. one having an oil free swept volume, is disclosed and described in our UK Patent Specification Ser. No. 2,088,957. The particular pump described therein can comprise a plurality, for example four pumping chambers, each containing intermeshing pairs of rotors to effect pumping action. Some of the chambers particularly the one at the pump inlet, can have rotors of the `Roots` type whereas other can have rotors of the `Claw` type. Shafts for driving the rotor pairs are interconnected by meshing gears enclosed in a housing at one end of the pump casing, one of the shafts extending beyond the housing for connection to a prime mover such as an electric motor. The housing, and the seals relating thereto, are such that oils or lubricants associated with the gears, etc. are prevented from leaking into the pumping chambers.
Oil free mechanical pumps of this type can generally provide a high volumetric pumping efficiency and are normally capable of evacuating an enclosure to a pressure of the order of 10.sup.-2 torr.
The absence of lubricant within the pumping chambers of such pumps makes them more suitable for applications where a dust or detritus laden gas has to be pumped from an enclosure. Any such lubricants present in the pumping chamber would act as a "scrubber" for such contaminants and can produce an abrasive slurry effective to induce rapid and excessive wear on the pump internal surfaces.
However, it has been found that in oil free mechanical vacuum pumps, particularly dry pumps such as are disclosed and claimed in the above UK patent, there can nonetheless be a progressive build up of contaminants such as dust and like detritus which arises mainly from the gas being evacuated or pumped from the enclosure. The processing of semi-conductor materials in the enclosure in particular, such as the coating of wafers of semi-conductor material, is known to produce significant quantities of such contaminants in the form of dust, detritus and the like.
It has also been found that over an extended period of pump operation, such contaminants can build up in a dry pump of the type with which this invention is concerned. It is thought that such build up can cause the pumps mechanically to seize for want of adequate clearance between relatively moving parts. Alternatively, particles of accumulated dust which have formed on the surfaces of the pump may break off in relatively large pieces and lock or trap the pump mechanism.
It has previously been proposed to overcome the problem of build up of contaminants by continuously introducing a dry, inert purge gas into a stage of the pump downstream of the inlet stage during the period of actual operation, i.e. with the pump evacuating an enclosure. This approach however suffers from the disadvantage that injection of such a purge gas adversely affects the volumetric pumping efficiency of the pump unless the purge gas injection rate is minimal; in such cases, however, the purging effect of the gas is substantially reduced and a little gain in purging is achieved at the cost of a loss of volumetric pumping efficiency.
The present invention is concerned with the provision of an oil free mechanical vacuum pump in which the build up of contaminants can be minimised.